Keywords: reinforced concrete shell, cooling tower, imperfection, wind load, elasto-plastic analysis.

A number of reinforced concrete (RC) towers constructed in the 1940s to 1970s are still used. In China and India, the construction of towers in power plants has increased and the scale of the tower has also enlarged. Investigations of the collapse accidents in the past and examination of the countermeasures are developing areas of study, and the evaluation of the safety of the existing facilities has become an important topic of research. With respect to the thin RC shells of these towers, much of the research has analysed the buckling-vibration problem and the effect of initial imperfections on structural properties. Several design guidelines have been proposed. To begin with, useful knowledge of the fundamental phenomenon and analysis method was obtained by studying the initial imperfections of a distribution idealized axisymmetrically and asymmetrically. Recommendations for the tolerance limit of the magnitude of the initial imperfections were proposed. According to the results of many leading research studies, the fluctuation in the in-plane stress in the meridian direction caused by the initial imperfections was inversely proportional to the shell thickness due to dead weight and wind load within the elastic stress analysis, and that it largely affects in-plane stress in meridian direction and bending stress in circumferential direction for the circumferential curvature error. In addition, partial initial imperfections with small amplitudes play a major role in meridian and circumferential stresses.

In this paper, three-dimensional geometric errors in a collapsed RC cooling tower shell (Ardeer Nylon Works, 1973) were reproduced and analyzed numerically in detail. Further, the effect of the geometric imperfection of the reproduced shape on the elastic stress distribution due to dead weight was examined, and the effect on the wind resistance performance was examined using elasto-plastic FE analysis.

The results of this study are as follows:

1. The reproduced geometric imperfection widely exists on the shell full face, and it becomes a waveform that recurs in the circumferential and meridian directions.
2. The hoop stress under the action of the dead load can be changed from tension to compression as a result of the imperfection.
3. Due to wind load, the bending cracks develop in the windward direction of the imperfect shell; this leads to the failure of the tower due to the tension that arises because of the bending in the meridian direction.
4. In the model with the imperfection, it is concluded that the turbulence in the remarkable section forces extends further than that in the case of a perfect shell. Moreover, the maximum reduction in the load bearing capacity is predicted to be approximately 18%. The effect of the difference between the wind directions is considered to be a remarkable result (The shape measurement value used in the analysis includes the effect of existing cracks, repetition of wind/thermal stresses and creep).
5. The effect of the comparatively small temperature loading on the lowering of the rigidity and the ultimate strength may be remarkable, and this fact is contrary to the usual behaviour of an RC member. In the case of a thin-shell cooling tower, the temperature crack may become a significant factor in the lowering of the ultimate strength. Therefore, this relationship needs to be examined prudently in the future.
6. In the design and safety assessment of existing towers, an elasto-plastic analysis seems to be necessary.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £145 +P&P)